GROOVY-11667: GPathResult: overload getAttribute and setAttribute

[eric-milles]

inner class dispatch is slightly different to support outer class member access

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.0250%. Comparing base (9299d69) to head (ce20f1b).
⚠️ Report is 733 commits behind head on master.

Files with missing lines	Patch %	Lines
...in/java/groovy/xml/slurpersupport/GPathResult.java	50.0000%	2 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2226        +/-   ##
==================================================
+ Coverage     69.0228%   69.0250%   +0.0023%     
- Complexity      29685      29686         +1     
==================================================
  Files            1423       1423                
  Lines          114342     114344         +2     
  Branches        19842      19842                
==================================================
+ Hits            78922      78926         +4     
+ Misses          28786      28785         -1     
+ Partials         6634       6633         -1     

Files with missing lines	Coverage Δ
...in/java/groovy/xml/slurpersupport/GPathResult.java	82.2335% <50.0000%> (-0.8434%)	⬇️

... and 2 files with indirect coverage changes

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